1. Field of the Invention
This invention relates to a process and apparatus for staged fuel combustion in connection with regenerative industrial furnaces, such as glass melting furnaces.
2. Description of Prior Art
An issue frequently encountered in connection with regenerative type industrial furnaces, such as regenerative glass melting furnaces, is the use of proper firing methods to fire fuel, that is, the burner firing method relative to combustion air port design. A properly selected firing method should provide the required flame length and shape for molten material coverage, uniform heat distribution, and low NO.sub.x emissions. Control of flame length and shape and uniform heat distribution are readily achieved by traditional firing methods, such as underport firing, side-of-port firing, overport firing, and throughport firing. The furnace and port design governs the method of fuel firing as well as the type of fuel utilized, that is, fuel oil or natural gas. However, maintaining NO.sub.x emissions at low levels using such traditional firing methods is not readily attainable, except when such traditional firing methods are combined with other NO.sub.x abatement technologies, such as ammonia injection which involves injection of a nitrogen-containing reagent, such as ammonia, into the high temperature combustion products, usually into the exhaust side of the regenerator checkers, in a narrow temperature window. NH.sub.2 produced by decomposition of the nitrogen-containing agent, reacts with NO to reduce it to N.sub.2. Ammonia injection typically reduces NO.sub.x by about 40%, but performance is limited by access to the exhaust gas region within the required narrow temperature window and NH.sub.3 slip.
U.S. Pat. No. 5,203,859 teaches another NO.sub.x abatement technology, namely oxygen-enriched air staging in which an oxygen-deficient primary flame, which inhibits NO.sub.x formation, and at the same time, custom engineered oxygen-driven ejectors installed at strategic locations in the furnace breastwall, provide oxygen-enriched air streams to effectively burn any unburned hydrocarbons and carbon monoxide within the furnace. Typically, NO.sub.x reduction can be anywhere from 50 to 75%.
U.S. Pat. No. 5,139,755 and U.S. Pat. No. 4,347,072 teach another NO.sub.x abatement technique, gas reburn. Gas reburn involves injection of natural gas into the furnace downstream of the conventional burners. The natural gas produces a fuel-rich combustion zone where NO.sub.x is reduced to N.sub.2 by hydrocarbon fragments. In addition, combustion air is added to burn out the combustibles. Under typical furnace conditions, gas reburn reduces NO.sub.x emissions by about 75%.
Yet another NO.sub.x abatement technology, oxy-fuel firing, is taught by U.S. Pat. No 5,199,866. The main focus of this concept is the removal of nitrogen from the combustion stream by using oxygen as an oxidant rather than air. However, the mechanics of 100% oxy-fuel firing on, for example, large float glass furnaces have not yet been demonstrated and extraordinary operating costs, for example the cost of oxygen use, associated with such a conversion remains to be a major impediment.
U.S. Pat. No. 4,403,941 teaches a combustion process for reducing NO.sub.x in combusters in which combustion takes place successively forming an incomplete combustion zone, a reducing combustion zone, and a complete combustion zone, respectively, corresponding to primary burners, secondary burners, and air ports or afterburners, successively arranged in the direction of gas stream in a furnace. U.S. Pat. No. 3,890,084 teaches a staged combustion process for reducing burner exhaust emissions in which burners in a vertically lower burner bank are initially fired with insufficient combustion air and burners in an upper burner bank are fired with additional combustion air to make up for the air deficiency of the lower burners, and, thus, provide all of the combustion air required for complete combustion of the fuel from the lower and upper burner banks.